This invention relates to the construction of liquid crystal display devices. In particular, the invention is concerned with a reflection-type liquid crystal display device and a transflective liquid crystal display device employing a single polarizing film method, for effecting bright display in black and white or in color by means of a reflector or a transflective reflector, and a sheet of polarizing film provided in a liquid crystal element of the device.
For a reflection-type liquid crystal display device, there has been mainly adopted a construction wherein a TN (twisted nematic) liquid crystal element or an STN (supertwisted nematic) liquid crystal element is disposed between a pair of polarizing films, and a reflector is installed on the outer side of one of the polarizing films.
With such a reflection-type liquid crystal display device, however, external light passes through each of two sheets of the polarizing films twice from the time when the external light enters from the visible side of the device until it goes out towards the visible side after reflected by a reflector, so that reduction in light quantity is increased, thereby lowering brightness of images in display. Moreover, since the reflector is installed on the outer side of a glass substrate of the liquid crystal element, there has arisen a problem that shadows appear on display.
To cope with the problem, a single polarizing film type liquid crystal display device, capable of effecting display with just one sheet of polarizing film, has since been proposed. With such a liquid crystal display device having only one sheet of polarizing film, reduction in light quantity can be decreased in comparison with the case of a conventional reflection-type liquid crystal display device employing two sheets of polarizing films, thereby improving brightness of images in display.
Further, with the a single polarizing film type liquid crystal display device, it is possible to solve the problem of the shadows appearing on display by forming a reflector inside a liquid crystal element.
Such a single polarizing film type liquid crystal display device is comprised of one sheet of polarizing film, one sheet of retardation film, and a liquid crystal element incorporating a reflector, as disclosed in, for example, Japanese Patent Laid-open No. H 4-97121 (JP, 04-97121, A).
With such a conventional single polarizing film type liquid crystal display device as described above, however, a problem has been encountered that excellent black display can not be effected, and contrast becomes low.
In order to effect excellent black display, a low reflectance (a ratio of an outgoing light quantity to an incident light quantity as seen from the visible side) needs to be achieved in black display parts at all wavelengths in the visible light region.
However, with the liquid crystal display device described above, explanation is given on its operation in the normally white mode wherein white display is effected in the xe2x80x9coffxe2x80x9d state when no voltage is applied to the liquid crystal element, and black display is effected in the xe2x80x9conxe2x80x9d state when a voltage is applied to the liquid crystal element, However, in the normally white mode, it is difficult to obtain excellent black display, so that nothing but display in low contrast is effected.
Accordingly, in order to obtain excellent black display, a reflection-type liquid crystal display device of the normally black mode was developed, wherein black display is effected in the xe2x80x9coffxe2x80x9d state when no voltage is applied to the liquid crystal element, and white display is effected in the xe2x80x9conxe2x80x9d state when a voltage is applied to the liquid crystal element, as disclosed in, for example, Japanese Patent Laid-open No. H 7-84252 (JP, 07-84252, A).
Even with such a liquid crystal display device as described above, a low reflectance for light rays over all wavelengths can not realized, because an optimization of xcex94nd value indicating a birefringent tendency of the liquid crystal element, placement angles as well as retardation values of retardation films, and placement angles of polarizing films are not sufficient, so that contrast is insufficient.
Furthermore, with the single polarizing film type conventional liquid crystal display device described in the foregoing, it is not possible to install a backlight because the reflector does not allow light rays to pass therethrough, so that display can not be seen at places where external light is weak or at night.
Accordingly, there has been developed a transflective liquid crystal display device, employing a half-mirror made of a thin film of aluminum formed by the vapor deposition method or the sputtering method, or having a reflector provided with an opening for every pixel, so that display is effected by light rays emitted from a backlight at places where external light is weak or at night.
In the case of the single polarizing film type liquid crystal display device, at the time of reflective display using external light when incident light passes through a liquid crystal element back and forth, the liquid crystal element and optical elements such as a retardation film, and so forth, need to be designed such that display in excellent black and white can be obtained by controlling outgoing of reflected light with a sheet of the polarizing film.
On the other hand, at the time of transmissive display using a backlight, since light emitted from the backlight passes through the liquid crystal element only once, the liquid crystal element and the optical elements need to be designed such that display in excellent black and white can be obtained in such a condition as described above by controlling outgoing of reflected light with one sheet of the polarizing film. For these reasons, it has been difficult to obtain high contrast in both reflective display and transmissive display.
A liquid crystal display device having a reflector provided with an opening for every pixel has been disclosed in, for example, Japanese Patent Laid-open No. H 10-282488 (JP, 10-282488, A), however, no description on the conditions concerning a liquid crystal element and optical elements has been given therein at all, and no description on how to achieve good contrast at the time in both reflective display and transmissive display has been given therein at all.
The invention has been developed in view of the technical background described above, and an object of the invention is to provide a single polarizing film type liquid crystal display device of to realize bright display in high contrast by obtaining excellent black display at low reflectance for light rays over all wavelengths.
Further, it is another object of the invention to provide a single polarizing film type liquid crystal display device, capable of effecting reflective display by use of external light and transmissive display by lighting up a backlight, and achieving high contrast at the time in both reflective display and transmissive display.
In order to achieve the objects described above, the liquid crystal display device according to the invention is a reflection-type liquid crystal display device of single polarizing film type which comprises: an STN liquid crystal cell comprised of a 200xc2x0 to 260xc2x0 twist-aligned nematic liquid crystal sandwitched between a first substrate having a reflector and first electrodes and a second substrate having second electrodes; a retardation film disposed on the outer side (a side opposite from the side facing the nematic liquid crystal) of the second substrate of the STN liquid crystal element; and further a polarizing film disposed on the outer side of the retardation film, wherein a xcex94nd value indicating a birefringent tendency of the STN liquid crystal element is in a range of 0.7 to 0.8 xcexcm, a retardation value R indicating a birefringent tendency of the retardation film is in a range of 0.35 to 0.40 xcexcm, and an intersection angle formed by a phase delay axis of the retardation film and an absorption axis or a transmission axis of the polarizing film is in a range of 30xc2x0 to 45xc2x0.
With the liquid crystal display device described above, reflective electrodes formed of a reflective material can be substituted for the first electrodes, doubling as the reflector, so that the reflector need not be installed separately.
Further, the liquid crystal display device according to the invention comprises an STN liquid crystal element comprised of a 200xc2x0 to 240xc2x0 twist aligned nematic liquid crystal sandwiched between a first substrate having a transflective reflector and first electrodes and a second substrate having second electrodes; a first retardation film disposed on the outer side (a side opposite from the side facing the nematic liquid crystal) of the second substrate of the STN liquid crystal element; a first polarizing film disposed on the outer side of the first retardation film; and a second retardation film, a second polarizing film, and a backlight that are disposed in sequence on the outer side of the first substrate of the STN liquid crystal element, thereby constituting a transflective liquid crystal display device.
With the liquid crystal display device described above, a xcex94nd value indicating birefringent tendency of the STN liquid crystal element is in a range of 0.7 to 0.8 xcexcm, a retardation value of the first retardation film is in a range of 0.35 to 0.40 xcexcm, an intersection angle formed by a phase delay axis of the first retardation film and an absorption axis or a transmission axis of the first polarizing film is in a range of 30xc2x0 to 45xc2x0, and a retardation value of the second retardation film is substantially equivalent to a quarter-wavelength.
With such a liquid crystal display device described above, it is preferable that a third retardation film is installed between the second retardation film and the second polarizing film such that a phase delay axis of the second retardation film crosses a phase delay axis of the third retardation film at about 60xc2x0, and a retardation value of the second retardation film is substantially equivalent to a quarter-wavelength while a retardation value of the third retardation film is substantially equivalent to a half-wavelength.
Otherwise, the phase delay axis of the second retardation film may cross the phase delay axis of the third retardation film substantially at right angles, wavelength-dependency of the retardation value of the second retardation film may differ from wavelength-dependency of the retardation value of the third retardation film, and the difference between the retardation value of the second retardation film and the retardation value of the third retardation film may be substantially equivalent to a quarter-wavelength.
Further, the transflective reflector may be a thin metal film with thickness in a range of 0.01 to 0.03 xcexcm or may be a thin metal film provided with an opening corresponding to respective pixels.
With each of the liquid crystal display devices as described above, a diffusion film is preferably installed on the outer side of the second substrate of the STN liquid crystal element.
Further, by installing color filters on a nematic liquid crystal side of the first substrate than the reflector of the STN liquid crystal element, or on the nematic liquid crystal side of the second substrate, the liquid crystal display device is turned into a liquid crystal color display device. If color filters are composed of filters in a plurality of colors, particularly three primary colors, full color display can be effected.
Furthermore, it is desirable that assuming that nx is refractive index in the direction of the phase delay axis of the retardation film, ny is refractive index in the direction orthogonal to the phase delay axis, and nz is refractive index in the direction of thickness thereof, refractive indices of the retardation film has a relationship of nx greater than nz greater than ny.
With the liquid crystal display device of the invention described above, the basic constitution of which is the same as that for the conventional reflection-type liquid crystal display device, or the conventional transflective liquid crystal display device of a single polarizing film type, however, the xcex94nd value of the liquid crystal element, the retardation value of the retardation films, placement angles of the retardation films, and placement angles of the polarizing films, were optimized in the normally black mode wherein high contrast can be obtained, by use of optical simulation and measured data.
As a result, it has been found that when no voltage is applied to the STN liquid crystal element in the case of reflective display, components of linearly polarized light at all wavelengths incoming through the polarizing film are turned into circularly polarized light upon passing through the retardation film and the STN liquid crystal element. The circularly polarized light is reflected at the reflector, and is transmitted again through the STN liquid crystal element and the retardation film, whereupon it reverts to linearly polarized light with the direction of polarization rotated by 90xc2x0, and is all absorbed by the polarizing film, so that perfect black display can be effected.
That is, by the agency of the polarizing film, the retardation film, and the STN liquid crystal element incorporating the reflector, black display when no voltage is applied is excellent, and bright white display can be effected because only one the polarizing film is adopted, thereby enabling display in high contrast to be effected. Further, no shadow occurs to display because the reflector is contained in the STN liquid crystal element.
Meanwhile, with the transflective liquid crystal display device according to the invention, in the case of transmissive display when the backlight is lit up, light emitted from the backlight is transmitted through the second polarizing film and the second retardation film having the retardation value equivalent to a quarter-wavelength that are installed on the underside of the STN liquid crystal element, and falls on the nematic liquid crystal after passing through the transflective reflector inside the STN liquid crystal element.
As described hereinbefore, since the retardation value and the placement angle of the first retardation film are optimized, a composite retardation value of the liquid crystal element and the first retardation film is rendered equivalent to a quarter-wavelength for nearly all wavelengths. Accordingly, by disposing the second retardation film on the underside of the STN liquid crystal element such that a retardation value thereof is subtracted by the retardation value of the STN liquid crystal element, birefringence can be eliminated, and the light emitted from the backlight is transmitted through the second polarizing film installed on the underside of the STN liquid crystal element, whereupon the light is turned into linearly polarized light with the direction of polarization rotated to the transmission axis of the second polarizing film, outgoing as it is from the first retardation film.
Accordingly, as the absorption axis of the first polarizing film on the visible side crosses the absorption axis of the second polarizing film on the side of the backlight at right angles, excellent black display can be effected.
In a state where a voltage is applied to the STN liquid crystal element, the birefringency of the STN liquid crystal element undergo changes, so that excellent white display can be effected at the time of reflective display as well as transmissive display, thereby enabling display in high contrast to be effected at both the time of reflective display and the time of transmissive display.